Changing riverine organic C:N ratios along the Pearl River: Implications for estuarine and coastal carbon cycles.

In the traditional view, riverine organic matter typically has a higher C:N ratio than marine phytoplankton 6.7:1 and has therefore been thought to be a carbon source in estuaries and coastal waters. Thus, a decrease in the riverine organic C:N ratio to <6.7:1 would potentially switch riverine organic matter from a coastal carbon source to sink. However, few studies have paid an attention to such a change. Our field investigation showed that organic C:N ratio was 11.8:1 in the pristine upstream section of a natural reserve, but decreased after the river passed through several urban cities, reaching 5.0:1 in near the Pearl River estuary. Along the river, dissolved inorganic nitrogen, total organic carbon and nitrogen all increased and they were highly negatively correlated with organic C:N ratios. The observation has a great implication that organic matter with a decreased C:N ratio from the Pearl River would potentially switch from a coastal carbon source of 2.8 × 10

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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.